1. Field of the Invention
The present invention relates to an optical fiber splicing method in which respective end faces of first and second optical fibers are fused and spliced to each other, and an optical fiber made by this method.
2. Related Background Art
Known as methods of connecting first and second optical fibers to each other are connector connection and fusion splicing. The fusion splicing, in which respective end faces of the first and second optical fibers are fused and spliced to each other, yields a lower splice loss and higher reliability than the connector connection does. Therefore, the fusion splicing is employed more often in general. In the case where the first and second optical fibers have respective mode-field diameters different from each other (e.g., one of them is a single-mode optical fiber whereas the other is a dispersion-compensating optical fiber) or in the case where the mode-field diameter of one of the first and second optical fibers is locally changed upon fusion, however, the splice loss therebetween is not sufficiently low.
Therefore, in the heating step after the splicing step, the dopant added to each optical fiber has been diffused, so as to lower the difference between mode-field diameters of the first and second optical fibers at their fusion-spliced part, or reduce the change in mode field diameter between the first and second optical fibers at their fusion-spliced part, thereby lowering the splice loss. Japanese Patent Application Laid-Open No. HEI 5-215931 and No. HEI 7-248423, for example, disclose that, in the case where the splice loss is measured simultaneously with the heating step, and the heating step is terminated at the time when thus measured splice loss becomes a predetermined value or less, the splice loss can reliably be reduced to a predetermined value or less.
However, the inventor has found that there is a case where the splice loss cannot be reduced to a predetermined value or less by the method disclosed in the above-mentioned publications. Namely, even if the heating step is terminated immediately after the measured splice loss attains a predetermined value or less, the splice loss measured upon heating immediately before the termination and the splice loss measured at room temperature after the termination will not always coincide with each other. Such a disparity is presumed to occur because of the fact that the thermal vibration of elements constituting each optical fiber at the time of heating differs from that at room temperature, so that the refractive index varies, thereby yielding mode field diameters different from each other. Due to such a disparity, the splice loss at room temperature is hard to decrease sufficiently.
In order to overcome the above-mentioned problem according to such inventor""s findings, it is an object of the present invention to provide an optical fiber splicing method and an optical fiber which can fully reduce the splice loss at room temperature.
The optical fiber splicing method in accordance with the present invention is a method of splicing a first optical fiber and a second optical fiber to each other by fusing respective end faces thereof, the method comprising (1) a splicing step of fusing the respective end faces of the first and second optical fibers and splicing them; (2) a heating step of heating a region including a fusion-spliced part between the first and second optical fibers fused in the splicing step; (3) a measuring step, alternately carried out with the heating step, of measuring a splice loss between the first and second optical fibers fused in the splicing step; and (4) a termination determining step of determining, according to a splice loss value measured in the measuring step, whether or not to terminate alternation of the heating and measuring steps. In the optical fiber splicing method in accordance with the present invention, the alternation is terminated when it is determined in the termination determining step that the alternation should be terminated.
In the present invention, whether or not to terminate the alternation of heating and measuring steps is determined according to the splice loss value measured in the measuring step. If it is resultantly determined that the alternation should be terminated, then the alternation is terminated, whereby the optical fiber connecting operation is terminated. As a consequence, the splice loss at room temperature is fully reduced.
In the optical fiber splicing method in accordance with the present invention, it may be determined in the termination determining step that the alternation should be terminated if the splice loss measured in the measuring step becomes a set value or less. Alternatively, it may be determined in the termination determining step that the alternation should be terminated if the difference between the splice loss measured in the measuring step and that at a previous measuring time becomes a set value or smaller.
In the optical fiber splicing method in accordance with the present invention, the termination determining step may be performed within a range of only a predetermined number of the alternation. This is suitable in cases where there are a plurality of values of number n of alternation at which the splice loss attains minimum values.
In the optical fiber splicing method in accordance with the present invention, a heating time in the first heating step may be made longer than that in the second and later heating steps. In this case, a desirable splice loss can be attained in a short time in total.
The optical fiber splicing method in accordance with the present invention may further comprise a change determining step of determining, according to the splice loss value measured in the measuring step, whether or not to change a heating condition in the heating step thereafter, and may change the heating condition in the heating step thereafter when it is determined in the change determining step that the heating condition should be changed.
In the optical fiber splicing method in accordance with the present invention, it may be determined in the change determining step that the heating condition should be changed if the splice loss measured in the measuring step becomes a set value or less. Alternatively, it may be determined in the change determining step that the heating condition should be changed if the difference between the splice loss measured in the measuring step and that at a previous measuring time becomes a set value or smaller.
In the optical fiber splicing method in accordance with the present invention, the change determining step may be performed within a range of only a predetermined number. This is suitable in cases where there are a plurality of values of number n of alternation at which the splice loss attains the minimum value.
In the optical fiber splicing method in accordance with the present invention, it may be determined in a plurality of change determining steps that the heating condition should be changed.
Preferably, in the optical fiber splicing method in accordance with the present invention, when it is determined in the change determining step that the heating condition should be changed, a heating temperature is lowered in the heating step thereafter, a heating time is shortened in the heating step thereafter, or a heating temperature is lowered while a heating time is shortened in the heating step thereafter.
The optical fiber splicing method in accordance with the present invention may further comprise an annealing step of cooling the fusion-spliced part to a temperature of 200xc2x0 C. over a period of at least 10 seconds after it is determined in the termination determining step that the alternation of the heating and measuring steps should be terminated and thus is terminated.
The optical fiber splicing method in accordance with the present invention may further comprise a thermal stress removal step of eliminating a thermal distortion by heat-treating a region including the fusion-spliced part at a predetermined temperature of at least 500xc2x0 C. but not higher than 1200xc2x0 C. while being lower than the heating temperature in the heating step after it is determined in the termination determining step that the alternation of the heating and measuring steps should be terminated and thus is terminated, and an annealing step of cooling the fusion-spliced part from the predetermined temperature to a temperature of 200xc2x0 C. over a period of at least 2 seconds after the thermal stress removal step.
In the optical fiber splicing method in accordance with the present invention, a region including the fusion-spliced part may be heat-treated in the annealing step.
In the optical fiber splicing method in accordance with the present invention, a region including the fusion-spliced part may be heat-treated in the thermal stress removal step while a heating source is relatively moved in a longitudinal direction of two optical fibers.
In the optical fiber splicing method in accordance with the present invention, the two optical fibers have a longitudinal temperature gradient of 500xc2x0 C./mm or less in the thermal stress removal step.
The optical fiber in accordance with the present invention is made by any of the optical fiber splicing methods mentioned above.